Radio Signal Strength Indication (RSSI) measurements in wireless communications systems are useful for a variety of reasons. RSSI measurements may be used in optimizing transmit levels for improved reception, conserving power due to transmit, and keeping transmit power low so as to avoid interference. Furthermore, RSSI can be used in ranging applications to make estimations on the distance between a transmitter and a receiver as the radio signal strength received typically decreases with increased distance from the transmitter.
When used for power control, a receiver can send commands to a transmitter if the RSSI gets too low or too high. The transmitter can then make any necessary adjustments to the transmit power. However, when RSSI is used in ranging applications, the transmit power must either be fixed or the relative changes known to make a range estimate. This is because the RSSI received is typically proportional to the transmitted power.
Because of multi-path interference (reflections), motion of receiver and transmitter, and a variety of other factors, a single RSSI measurement is usually not accurate. One way to improve accuracy is to take multiple RSSI samples and average them. Another way to reduce errors due to multi-path is take the RSSI samples at different frequencies. Multi-Frequency systems (hopped or other-wise) already change frequencies to optimize reception for this very reason. By taking RSSI measurements at each frequency and averaging these, one can get a better estimate of range as the individual measurements may have different results depending on frequency.
Many wireless devices have both an “active mode” and a “low power mode” (also referred to herein as “reduced power mode”) of operation. In low power mode, the wireless device reduces operation in some manner to conserve power. In active mode, taking multiple RSSI samples as desired does not present a problem because the measurements are a small burden relative to the power intensive time consuming process of maintaining an active link. The term “active mode” includes the full power mode of operation of the device, but may also refer to any mode of operation in which the device consumes more power than the “low power mode”.
However, when a wireless system goes into low power mode, unnecessary operations are canceled in order to reduce power. The system cannot always be RSSI sampling as this takes time and power. RSSI averaging is essential in frequency hopped systems as any given channel may be significantly worse than the overall path loss due to distance. Typically the system will wake-up, take a measurement and go back to sleep. For example, if the system requires 100 samples in its average, there can be a very long settling time to a step change on the average RSSI value in low-power mode compared to the active mode settling time (and faster sample rate) if the time between wakeups is long. There will also be long delays while a change in transmit power is integrated into the RSSI estimate. For example, in a system where the system averages a pre-determined number of measured RSSI values to determine an accurate RSSI value, the settling time is the time it would take all of the samples to be flushed through the averaging filter once a change in RSSI has been detected (e.g., due to movement of the mobile device). In low-power mode, the settling time is relatively long due to the increased length of time between measurements. In contrast, the settling time for the averaging filter is much faster in active mode where RSSI measurements are made with increased frequency. In a further embodiment, the settling time may be the time it takes for the mobile device to retrieve/measure a pre-determined number of RSSI measurements. In this case, the settling time is much longer for the low-power mode than the active mode for the same reasons.
In the prior art, one way to reduce the settling time has been to reduce the number of RSSI samples averaged. However, while this speeds up the process, it also means that the average value will be fairly inaccurate due to the reasons that motivated averaging in the first place. Another solution is to reduce the time between low-power samplings. However, the power savings of low power mode is based on duty cycling, where the duty cycle is the ratio of time to time between wake-ups. Thus, reducing sample time undesirably increases power consumption during the low-power state.
As a result, improved methods and apparatuses are needed for RSSI measurements.